Optical films that exhibit a visible color shift as a function of viewing geometry are known. See, e.g., PCT Publication WO 99/36258 (Weber et al.) entitled “Color Shifting Film”. See also U.S. Pat. No. 6,045,894 (Jonza et al.) entitled “Clear to Colored Security Film”. These references disclose many different films, each of which exhibits a shift in apparent color as the observation or incidence angle θ (measured from the surface normal) changes. Filters that comprise a glass or other rigid substrate having a stack of inorganic isotropic materials deposited thereon can also exhibit color shifts.
A common feature of these films is the presence of one or more reflection bands for normally incident light (θ=0), which band(s) then shift to shorter wavelengths as θ increases. The physics of this so-called “blue shift” of the reflection band can be explained in connection with FIG. 1, where a portion of a multilayer film 10 is shown greatly enlarged. A light ray 12 is incident from medium 1 (with isotropic refractive index n1, for simplicity) at an angle θ1. Part of the light ray reflects at an upper interface 14 between medium 1 and medium 2, and another part reflects at a lower interface 16 after traversing the layer of medium 2, whose physical thickness is d. Medium 2 is also assumed to have an isotropic refractive index, n2, for simplicity. The two reflected rays 18, 20 eventually constructively or destructively interfere depending on the relative phases of the rays. The relative phase in turn is a function of the optical path difference (OPD) between the rays, given by:OPD=2·n2·d·cos(θ2)  (EQ. 1)This quantity decreases with increasing incidence angle, corresponding to a shift to shorter wavelengths. Although the analysis is more complicated, multilayer optical films that have at least some optical layers that are birefringent rather than isotropic also experience a blue shift with increasing angle.
As the band(s) shift to shorter wavelengths, they also each split into two distinct bands: one for s-polarized light, the other for p-polarized light, where s-polarized light refers to linearly polarized light whose electric field vector oscillates perpendicular to the plane of incidence, and p-polarized light refers to linearly polarized light whose electric field vector oscillates parallel to the plane of incidence. The shift to shorter wavelengths can also be accompanied by a shift in the spectral width and shape of the reflection band, and changes in the out-of-band and in-band reflectivity. The amount of blue-shift one can attain is limited, and is a function of the medium in which the film is immersed, and the details of the film construction.
The observed color change of these known films is a manifestation of the shift of the reflection band(s) to shorter wavelengths. Since the human-visible region corresponds to a segment of the electromagnetic spectrum extending from about 400 to 700 nm, a film that is clear (i.e., substantially colorless) at normal incidence can become colored at oblique angles only by the shifting of a reflection band whose position at normal incidence is somewhere in the near infrared region, i.e., at or above about 700 nm. As this band begins to move into the visible region with increasing observation angle, it begins to block long visible wavelengths in the red, thus giving rise to a cyan appearance in transmission. This is shown schematically in FIG. 2, where a reflection band 30a for normally incident light is located initially in the near infrared region of the spectrum, and then as the angle of observation increases it transforms into band 30b at shorter wavelengths, and with increasing observation angle transforms into band 30c at still shorter wavelengths. (In FIG. 2, spectral ringing and separation into distinct s- and p-polarization reflection bands are ignored for ease of explanation.)
The spectral position of the reflection band at normal incidence is set by the optical thickness of the optical repeat units in the film. The optical thickness of a layer refers to its physical thickness multiplied by the relevant refractive index of light. Optical repeat unit refers to a stack of at least two individual layers that repeats across the thickness of a multilayer optical film, though all repeating layers need not have the same thickness. As an example, known clear-to-colored films reflect normally incident light from approximately 720 to 900 nanometers by utilizing optical repeat units whose optical thicknesses range from 360 to 450 nanometers (half the wavelength of the light desired to be reflected).
It would be advantageous to have at the optical designer's disposal films that could exhibit human visible color shifts other than those caused by a simple blue shift of existing reflection bands. Further, it would be advantageous to have available a film that could transition from clear at normal viewing to any desired color at an oblique angle.